1. Field Of The Invention
The present invention relates to innerducts utilized in the communications industry for the passage of fiber optic cables therethrough. More particularly, the present invention relates to a coextruded innerduct, having a filled outer layer so that a substantially rigid length of innerduct can be positioned within an outer casing of dissimilar material.
2. General Background
In the communications industry, the use of fiber optic cables, as a transmission means for telephone lines, has become quite common, in both underground or overhead installation. The fiber optic cables which are quite small in diameter, are usually housed within an innerduct of PVC material, which in turn would be positioned within an outer casing of PVC so that the fiber optic cables are protected from the elements when installed underground or over bridge crossings or the like.
At present there are two principal systems for installing fiber optic cables. The first system would be entitled the use of reeled innerduct, and the second system would involve the use of a multi-cell system. Both systems, however, have suffered from the problem of the friction which occurs between the inner wall of the innerduct and the outer wall of the fiber optic cable as the cable is being fed through the innerduct. Often times the innerduct is laid in lengths of thousands of feet, and the fiber optic cable, because of it being a continuous type cable, must be fed or blown through the innerduct in order to complete the installation. However, because of the friction which is involved between the walls of the innerduct and the fiber optic cable over such a distance, this is a difficult thing to achieve at times.
In order to solve the problem of reducing the friction between the innerduct and the fiber optic cable, in the art, there have been introduced the use of lubricating materials such as glycol or other water soluble polymeric lubricants incorporated into the interior wall of the innerduct itself so as to reduce the factor of friction as the cables are moved through the innerduct into position.
There has been granted a patent in the art, Shoffner U.S. Pat. No. 4,892,442, entitled "Prelubricated Innerduct" and assigned to Dura-line, Inc., which discloses an innerduct which is formed by coextruding an outer jacket of high tensile strength polymer and an inner barrier layer of highly lubricous polymer. The intent is to provide an innerduct which has an inner layer impregnated with lubricous material so that in the event there is wear of the tube's inner walls during cable placement, the wear will only expose further lubricants and not increase friction between the respective elements during the duct or cable replacement operation.
The inner core of the material impregnated with the lubricous agent lowers the coefficient of friction to other objects such as the fiber optic cables being pulled through the innerduct. The outer jacket is formed of high tensile strength polymers with a high molecular weight, such as high density polyethylene, so that the innerduct may be formed or wrapped about a spool or the like for subsequent transport. The reeled or rolled innerduct, as it is commonly referred to in the art, would then be continually fed from the roll into placement prior to the running of the fiber optic cable therethrough. This patent, therefore, apparently sought to solve the problems of a) providing a lubricated surface by co-extruding a lubricous agent to lower the coefficient of friction between the surface and the fiber optic cable being threaded therethrough, and b) to provide a high tensile strength exterior layer which would allow the innerduct to be formed around a spool so that it could be placed at the location directly from the spool.
Other problems which the '442 patent did not address are problems confronted in the use of this type of innerduct in straightway, as part of a multi-cell application. For example, in the area of laying fiber optic cables, the alternative to the use of the uninterrupted pathway of rolled innerduct, is through the use a multi-cell system which would comprise, in general, a plurality of lengths of innerduct which would be interconnected via a coupling body, such as the coupling body disclosed and claimed in U.S. Pat. No. 4,834,825, which would couple multiple twenty-foot sections of innerduct end-to-end in order to form a multi-duct configuration for laying the fiber optic cable therethrough. The multiple sections of innerduct placed end to end with a coupling body spaced at twenty foot intervals would be contained within an outer casing of PVC pipe, so as to provide an outer protective layer over the coupled sections of innerduct.
The problems in attempting to utilize the innerduct as claimed in the '442 patent in a multi-cell application are at least two-fold. First, the polyethylene innerduct is of a different molecular structure than the PVC outer casing, and therefore the fluctuations in temperatures under different weather conditions results in the innerduct and the outer casing expanding and contracting in different amounts. Therefore, although the innerduct and the casing ends may be flush when they are placed into position, over a certain period of time, due to changing weather conditions, the innerducts may have expanded to where they protrude out of the outer casing, which is undesirable in the coupling process between another section of casing and innerduct, resulting in damage to the ends of the innerducts.
The second problem is the fact that the innerduct, as claimed in '442 has a high tensile strength outer layer which results in the innerduct being specifically constructed to curve or bend, and sustain loads required to pull the duct into long lengths of outer duct, which is undesirable in a multi-cell setting. It is imperative to have all twenty foot sections of innerduct very rigid so as to form a straight twenty foot multi-cell group of innerducts which can be easily positioned into the outer casing of PVC pipe.